operating characteristics of induction motor revised

7/30/2019 Operating Characteristics of Induction Motor Revised

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Chamila Sumathiratna-FIVT


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Locked rotor torque

Pull up torque

Break down torque

Full load torque

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When the resistance of the rotor bars are loweredthe pullout torque comes closer to synchronousspeed


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High efficiency at normal operating conditions requires a low rotorresistance. On the other hand, a high rotor resistance is required to produce a

high starting torque and to keep the magnitude of the starting currentlow and the power factor high.

The wound rotor is one way of meeting the above mentioned need for

varying the rotor resistance at different operating conditions. Wound-rotor motors are, however, more expensive than squirrel-cagemotors.

.

Effect of the rotorresistance

the torque-slipcurves


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Following double squirrel-cage arrangements can also be used toobtained a high value of effective resistance at starting and a low value ofthe resistance at full-load operation.

It consists of two layers of bars, both short-circuited by end rings.

The upper bars are small in cross-section and have a high resistance.

They are placed near the rotor surface so that the leakage flux sees a pathof high reluctance; consequently, they have a low leakage inductance.

The lower bars have a large cross-section, a lower resistance and a highleakage inductance.

Double squirrel-cage rotor bars


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At starting, rotor frequency is high and very little current flows throughthe lower bars; the effective resistance of the rotor is then the highresistance upper bars.

At normal low slip operation, leakage reactance are negligible, and therotor current flows largely through the low resistance lower bars; the

effective rotor resistance is equal to that of the two sets of bars in parallel.

Double squirrel-cage rotor bars


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The use of deep, narrow rotor barsproduces torque-slip characteristicssimilar to those of a double-cage rotor.

Leakage inductance of the top cross-section of the rotor bar is relatively low;

the lower sections have progressivelyhigher leakage inductance.

At starting, due to the high rotorfrequency, the current is concentratedtowards the top layers of the rotor bar.

At full-load operation, the currentdistribution becomes uniform and theeffective resistance is low.


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At low slip rotor frequency is low therefore lessleakage reactance allow to conduct currentduring running

At high slips reactance are large compared to

resistance so the current is forced through thebars at surface which reactance is lowcompared to deeper bars.

These conductors closer to surface have higherresistance

A high torque at starting could be obtained

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Class A

Standard motor design

Full load torque and pull out torque occurs at

low values of slip High inrush currents 500% to 800% rated

current

Applications-fans , blowers , pumps etc.

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Class B

Normal starting torque

Same starting torque as class A

Applications are similar to class A

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Class C

Has high starting torque

Double cage motors

Applicationsneed high starting torque like

compressors and conveyors

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Class D

Very high starting torque

Pullout torque occurs at low slip

Used for high inertia loads like fly wheels andpunch press

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Why need a starting mechanism?

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Direct On Line/Full Voltage starting/Acrossthe line starting

Star Delta

Auto-Transformer Primary Resistance Reduced Voltage

Rotor Resistance

Electronic Soft Start

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apply full line voltage to themotor upon starting

when the high startingcurrent does not affect the

power supply system andthe machinery will standthe high starting torque

manual or magnetic. When the supply voltage

fails, the motor will stopand restart automaticallywhen supply isrestored(manual)

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Double Pole manualStarter

Three pole manual

starter


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Unlike themanual starterin which thepower contactsare closedmanually ,themagneticstarter motor

starter contactsare closed byenergizing aholding coil.

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Resistors inserted into theprimary of the motor increasethe overall impedance of thecircuit and reduce the starting

current. In effect the voltagedrop across the resistors givesa reduced voltage to the motorterminals. While reducing thecurrent, we should alsoremember that the torque willalso be reduced. than for astar-delta starter


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the transformer is connected instar (-KM1) and the linecontactor (-KM2) closed. Thisapplies a reduced voltage forthe initial starting of the motor.

when the motor has reached fullspeed the auto-transformer starcontactor (-KM1) is opened.This momentarily places thetransformer in-line with themotor (as an inductance), untilstep 3 is complete.

the line contractor (-KM3) isclosed putting the line voltageonto the motor and the auto-transformer isolated bycontactor (-KM2) opening.

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KM1 is initiallyclosed to put themotor in starconfiguration sametime KM2 also closed

At the end of startingKM1 is opened and

KM# is closed toplace the windings indelta


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Soft starters use acombination ofpower electronicsand electroniccontrol circuitryto slow increasethe voltage on themotor duringstarting; ensuinga smoothacceleration.

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Spikes atreducevoltagestart

Voltagelimiting

Currentlimiting


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Electronic soft starters contain the thyristors controlthe firing

Modern soft starters can set varying start and stop

ramps, setting of the initial starting voltage, currentlimiting control and thermal overload protection.firing of thyristors other than a zero voltage (current)will create a non-linear load characteristic

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Soft start

Current limit start

Full-voltage start

Linear speed acceleration

Preset slow speed

Soft Stop

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operating characteristics of induction motor revised

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